And it's not like stealing a car, it's like standing on a different hillside looking at the same sunset. There's enough sunset to go around.

On a more practical level, fan writers who care about canon often spend enormous amounts of time and money and effort to make sure they have the source material -- buying full sets of tapes and DVDs of a series, and then more tapes and DVDs of other performances by actors who were in the series. The producers and studios do not lose out monetarily from the devotion of fans; they sell much more than they might otherwise sell.